System and method for joining workpieces to form an article

ABSTRACT

Workpieces are placed in the workstation so one is in registration with a fixed locator. Measurement data are obtained reflecting the positions of the respective features of the workpieces and represented in a common reference frame associated with the fixed locator. A processor uses the collection of assembly data and the measurement data to define and store ordered pairs of mating feature locations. The pairs are then reoriented using a computationally discovered best fit. The position of a feature demarked for registration with the adjustable locator is calculated and the adjustable locator is caused to move to the calculated position of the feature demarked for registration thereby establishing a best fit orientation of the mating workpieces in physical space. The mating workpieces are then positioned in said best fit orientation by registration with said fixed and adjustable locators and then mechanically joining the mating workpieces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/208,124, filed on Aug. 21, 2015. The entire disclosure of the aboveapplication is incorporated by reference as if fully set forth in detailherein.

FIELD

The present disclosure relates to a system and method for joiningworkpieces to form an article.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

It is relatively common in the manufacture of articles, such asautomobile bodies, printed circuit boards, and aircraft fuselages, tosecure components to fixtures and then to assemble the componentstogether. The fixtures are intended to orient the components relative toone another, in a manner that attempts to position selected features onthe components at a nominal position. This practice has significantdrawbacks including dimensional variability leading to quality issues,high scrap and a reduced ability to compete in a global market. As anexample, variation reduction in the assembly of automotive bodies is achallenging endeavor. A typical automobile body can employ between 100to 150 sheet metal parts, which can be assembled to one another in amodern, high volume assembly line in 55 to 75 assembly stations. Theassembly line can have between 1500 to 2000 fixture locators that areemployed to locate various combinations of the components to performapproximately 4000 welds. A single failure that occurs at a locator, awelding spot or from a part that is not fabricated correctly can cause adimensional variation. This variation will then propagate downstreamfrom station to station and accumulate at the final station of the bodyassembly. After fabrication/assembly of the body, the vehicle doors,hood, windshield, and trunk lid are then mounted and fit to the body.The dimensional variation is accumulated at the body assembly openings,compounded by variations associated with panels or other subassemblies,and will significantly increase the manufacturing complexity, leading tomore tooling failures and unexpected downtime and reduction in bothproduct quality and production throughput. These issues can also causeleaks, noise and risk of water leakage. While there has been muchaccomplishment in the art of data acquisition using laser sensors andoptics, little has been done to utilize the data in real time to impactthe dynamic movement of the tooling to reduce variation. Mostmanufacturing companies continue to rely on human knowledge and analysisto estimate and make tooling corrections (shimming) one locator at atime—after the fact, meaning parts have already been incorrectly joined.It's worthy to note that dimensional problems contribute toapproximately two thirds of all quality-related problems during newproduct launches.

One drawback relates to variation in the manufacturing processes for thebody components and the inability of the prior art assembly practice toimmediately adjust to the particular tolerance/configuration of a bodypart loaded into the weld fixture.

Conventional weld fixtures commonly employ locator pins that engageholes in one or more of the body components. As many of the bodycomponents are stampings, the diameter of the holes tends to berelatively consistent, but due to variation in the material from whichthe body components are formed (e.g., chemical composition,micro-structure, thickness of the material) and variation in theprocessing of the material (e.g., temperature of the stamping die,type/condition of lubrication, amount of lubrication, temperature ofworkpiece, speed in which the body component is formed), the exactlocation of these holes tends to move (albeit in a small manner)relative to other features on the body component. It should beappreciated that while the variation is relatively small, the variationof each of the features can compound when the body component is joinedto other body components.

Despite the existence of part-to-part variation in a body component, itis common practice in the assembly plants to employ nominal partgeometry and/or historic sample data to position a locator relative to aframe of a weld fixture and thereafter employ trial-and-error techniquesto reposition (i.e., shim) the locator as needed. Moreover, in anattempt to eliminate small positioning errors between the weld fixtureand the body component (which would further contribute to undesiredvariation in the joining of the body components together), the locatorpins are commonly sized to the largest possible or largest statisticallyprobable diameter of the hole in the body component. Consequently, it isnot uncommon for the locator pin to engage the hole in the bodycomponent in an interference fit manner. Moreover, it is not uncommon,due to the variation in the positioning of the hole(s) in the bodycomponent relative to other features, to have the locator pin offsetfrom the hole in the body component (i.e., the axis of the locator pinis offset from the axis or centerline of the hole in the bodycomponent). Accordingly, a relatively large amount of stress can beexerted on the body component and the locator pin as a result of theinterference between the locator pin and the hole in the body component,and the offset between the axis of the locator pin and the centerline ofthe hole in the body component. These stresses induce wear on thelocator pins, which can essentially machine the locator pins to asmaller diameter, and can cause the locator pin to fail.

In view of the above remarks, there remains a need in the art for animproved process for fabricating a multi-piece assembly that is formedby joining multiple components together.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In accordance with one aspect, a method is disclosed for forming amulti-piece assembly in a plurality of joining operations thatprogressively joins workpieces together, each of the workpieces havingone or more features, the multi-piece assembly having an assemblyspecification that defines target positions of the features of theworkpieces relative to a datum. According to the method first and secondworkpieces are secured to at least a first assembly fixture. An initialpositioning is determined between the first and second workpieces thatrelates a first three-dimensional data set of the first workpiece to asecond three-dimensional data set of the second workpiece. The firstthree-dimensional data set includes three-dimensional data on each ofthe features on the first workpiece; the second three-dimensional dataset includes three-dimensional data on each of the features on thesecond workpiece. A desired relative positioning is determined betweenthe first and second workpieces based at least partly on the first andsecond three-dimensional data sets and the assembly specification. Thedesired relative positioning is specific to a given pair of the firstand second workpieces and is configured to locate each of the featuresof the first and second workpieces in such a way that discrepancies inthe positioning of all of the features on the first and secondworkpieces relative to their positioning in the assembly specificationare minimized in a desired manner.

While the first and second workpieces are secured in the at least afirst assembly fixture, the first workpiece is repositioned relative tothe second workpiece such that the first and second workpieces are inthe desired relative positioning; and the first and second workpiecesare then together.

According to a further aspect the joined first and second workpiecesform a sub-assembly and the method further includes securing thesub-assembly and a third workpiece to at least a second assemblyfixture; determining an initial positioning between the sub-assembly andthe third workpiece that relates the first and second three-dimensionaldata sets to a third three-dimensional data set of the third workpiece,the third three-dimensional data set including three-dimensional data oneach of the features on the third workpiece; determining a desiredrelative positioning between the sub-assembly and the third workpiecebased at least partly on the first, second and third three-dimensionaldata sets and the assembly specification, wherein the desired relativepositioning between the sub-assembly and the third workpiece is specificto a given pair of the sub-assembly and the third workpiece and isconfigured to locate each of the features of the third workpiecerelative to the features of the sub-assembly in such a way thatdiscrepancies in the positioning of all of the features on the first,second and third workpieces relative to their positioning in theassembly specification are minimized in a desired manner; while thesub-assembly and the third workpiece are secured to the at least thesecond assembly fixture, repositioning the third workpiece relative tothe sub-assembly such that the sub-assembly and the third workpiece arein the desired relative positioning between the sub-assembly and thethird workpiece; and joining the sub-assembly and the third workpiecetogether.

According to another aspect, a method is disclosed for forming amulti-piece assembly by joining mating workpieces in a workstationhaving at least one fixed locator and at least one adjustable locator,wherein each workpiece has at least one physical feature, the positionof which within the multi-piece assembly is expressed in a collection ofassembly data. The workpieces are placed in the workstation so that oneof said workpieces is in registration with the fixed locator.Measurement data are obtained reflecting the positions of the respectivefeatures of the workpieces and representing the measurement data in acommon reference frame associated with the fixed locator.

A processor is then used to ingest and utilize the collection ofassembly data and the measurement data to define and store in memoryordered pairs of mating features. The processor is used to define andstore in memory, for each ordered pair of mating features, a test vectorthat connects a pair of mating features, where each test vector hasvector components represented as variables stored in memory. Theprocessor is then used to computationally discover a best fit for themating workpieces by collectively applying to the test vectors acomputational optimization process that seeks to minimize the lengths ofthe test vectors while taking into account predefined weighting factorsthat give certain features locational preference over others.

The processor then reorients the data representing the feature locationsof the respective workpieces in the common reference frame using thecomputationally discovered best fit. The processor then calculates theposition in the common reference frame of a feature demarked forregistration with the adjustable locator. The adjustable locator is thencaused to move to the calculated position of the feature demarked forregistration with the adjustable locator and thereby establishing a bestfit orientation of the mating workpieces in physical space. When this isdone, the mating workpieces are positioned in said best fit orientationby registration with said fixed and adjustable locators and thenmechanically joining the mating workpieces.

In yet another aspect, method is disclosed for forming a multi-pieceassembly by joining mating workpieces in a workstation having at leastone fixed locator and at least one adjustable locator, wherein eachworkpiece has at least one physical feature, the position of whichwithin the multi-piece assembly is expressed in a collection of assemblydata. The workpieces are placed in the workstation so that one of saidworkpieces is in registration with the fixed locator. Measurement dataare obtained reflecting the positions of the respective features of theworkpieces and representing the measurement data in a common referenceframe associated with the fixed locator.

A processor is then used to ingest and utilize the collection ofassembly data and the measurement data to define and store in memoryordered pairs of mating feature locations. The processor is used toreorient the pairs of mating feature locations using a computationallydiscovered best fit. The processor is then used to calculate theposition in the common reference frame of a feature demarked forregistration with the adjustable locator. The adjustable locator is thencaused to move to the calculated position of the feature demarked forregistration with the adjustable locator and thereby establishing a bestfit orientation of the mating workpieces in physical space; The matingworkpieces are then positioned in said best fit orientation byregistration with said fixed and adjustable locators and thenmechanically joining the mating workpieces.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic illustration of an exemplary manufacturing systemconstructed in accordance with the teachings of the present disclosure;

FIG. 2 is a perspective view of a portion of the manufacturing system ofFIG. 1, illustrating an exemplary positioning fixture constructed inaccordance with the teachings of the present disclosure;

FIG. 3 is a schematic view depicting a portion of the positioningfixture of FIG. 2;

FIG. 4 is a schematic view depicting a portion of the positioningfixture of FIG. 2;

FIG. 5 is a schematic illustration of an exemplary process for joiningworkpieces in accordance with the teachings of the present disclosure;

FIG. 6 is a schematic illustration of an exemplary assembly that isformed of workpieces that are to be joined in a workstation of themanufacturing system of FIG. 1;

FIG. 7 is an enlarged portion of FIG. 6 that depicts a first orientationof selected features on a workpiece and a revised orientation of theselected features that is obtained through an optimization algorithm;

FIGS. 8 and 9 are schematic depictions of workpieces that are to bejoined in the manufacturing system of FIG. 1, the workpieces beingdepicted as having independent and dependent relationships,respectively;

FIG. 10 is a plot that depicts the propagation of variation in thefabrication of an article;

FIG. 11 is an exemplary hardware diagram also illustrating anoptimization algorithm by which the position of the locators and/orrobotic positioners is driven;

FIG. 12 is a data structure diagram useful in understanding how theprocessor can be programmed to store and manipulate data used tomechanically position the locators and/or robotic positioners;

FIG. 13 is a vector diagram useful for understanding how theoptimization algorithm of FIG. 11 manipulates connected relationshipsamong features of respective workpieces; and

FIG. 14 is a vector diagram useful for understanding the optimizationalgorithm.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Contrary to conventional manufacturing practice, where fixed locatorscontrol physical relationship among workpieces to be joined, thedisclosed system and method takes the actual, measured geometricproperties of each workpiece into account and uses a processor tocontrol a variable locating and positioning system so that theworkpieces may be assembled in an optimal way to form the finishedarticle. As will be more fully explained, the measured geometricproperties of each workpiece can be measured at the assemblyworkstation, or they may be measured in advance and supplied as datawith each workpiece. The processor can be implemented either by theassembly workstation controller or by a separate computer thatcommunicates with the controller.

Specifically, the processor is supplied with data describing how theworkpieces are to be assembled into the finished article. This can beperformed by supplying the processor with computer-aided design (CAD)data or by using a master component with high dimensional accuracy. Theprocessor uses the supplied data and workpiece measurement data tocompute an optimal solution whereby these specifically measuredworkpieces can be joined according to the supplied data (e.g., CADdata). As will be explained, the optimization algorithm executed by theprocessor takes predefined weighting factors into account, to cause thepositions of certain points to more stringently adhere to the supplieddata than other points. Thus, for example, in the case of an automobileassembly, joining the underbody to the side of the car or the fitbetween the passenger door and frame can be made more precise, at theexpense of permitting a looser fit between other components where aprecise fit is of little consequence.

As will be more fully explained, using the optimization algorithm, theprocessor calculates the optimal way to fit a particular, measured setof workpieces together. The processor then determines where thepositioning fixture locators of a workstation need to be positioned toallow the workpieces to be assembled according to the determined optimalfit. The processor then sends control signals to cause these locatorsand/or the arms of dynamic positioners (e.g., positioning robots) toplace the workpieces in their proper position so that they may bepermanently joined. To better appreciate how the optimization algorithmcan improve the workpiece joining process, a discussion of an exemplarymanufacturing process will now follow.

With reference to FIG. 1 of the drawings, an exemplary system formanufacturing an article is generally indicated by reference numeral 10.The manufacturing system 10 can include a plurality of workstations thatare employed to form one or more portions of the article from variousworkpieces. The workstations can be collectively or generally designatedby reference numeral 12, or can be referred to specifically by referencenumerals 12(a), 12(b), . . . 12(n−1) and 12(n). Each of the portions ofthe article can comprise two or more workpieces (e.g., W1, W2) that havebeen joined together in one of the workstations 12. In the particularexample provided, the workpieces form a lap joint that is joined orsecured by welding, but those of skill in the art will appreciate thateach workstation 12 could employ one or more joining techniques, such aswelding (e.g., arc-welding, MIG welding, TIG welding, spot welding,resistance welding), bonding, riveting, fastening, nailing, brazing,soldering, etc. Additionally, those of skill in the art will appreciatethat the particular joint need not be a lap joint, but could be any typeof joint, including a butt joint or a mixed joint. Each workstation 12can comprise a positioning fixture 20, a measuring device 22, and aworkstation controller 24. As indicated above, the optimizationalgorithm for determining proper locator positions may be performed bythe processor (not shown in FIG. 1) within controller 24, or by anauxiliary computer (not shown in FIG. 1).

With reference to FIG. 2, each positioning fixture 20 can be configuredto hold one or more workpieces (W1, W2) so that the joining process canbe conducted in the workstation 12. The positioning fixture 20 can havea frame (jig) 30 and a plurality of locators 32 that are configured toposition one or more of the workpieces relative to the frame 30.

Depending on the configuration of the workpieces that are to be joinedin the workstation 12, the locators 32 can be configured in one or moresets. Generally speaking, one set of locators 32 can be employed tolocate a first workpiece relative to the frame 30 and another set oflocators 32 can be employed to locate a second workpiece relative to theframe 30. It will be understood, however, that one or more of thelocators 32 in one set of locators 32 could be employed to position twoor more workpieces relative to the frame 30. As will be discussed inmore detail below, the locators 32 will include at least one primarylocator 32 p. In some situations, a first workpiece can be positioned ina given workstation with a set of locators having no primary locators 32p, and each workpiece that is to be joined to the first workpiece in thegiven workstation can be positioned with a set of locators having twoprimary locators 32 p. The locators 32 can be configured to engage oneor more of the workpieces in a controlled manner to both locate theworkpiece(s) in a desired manner and to eliminate translating movementalong and rotational movement about X, Y and Z axes. One techniquecommonly employed in the design of positioning fixtures aims toconstrain the degrees of freedom of a workpiece during a joiningoperation to ensure repeatable build. Such system is referred to as the3-2-1 principal of fixture design. According to this design, thepositioning fixture 20 is configured such that each workpiece:

-   -   a) rests on three non-collinear net contact blocks on a bottom        surface (i.e., in an X-Y plane), which fixes the location of the        workpiece in a first direction along the Z-axis, rotationally        about the Y-axis, and in a first rotational direction about the        X-axis;    -   b) rests on a four-way locating pin on a side (X-Z plane), which        fixes the motion of the workpiece in both X-Y plane one        direction along the X-axis and another around the Y-axis and in        a first rotational direction about the Z-axis; and    -   c) rests on one two-way locating pin on an adjacent surface (Y-Z        plane) to fix the location of the workpiece in one direction        along the Y-axis and in a second rotational direction about the        Z-axis. Additionally, it will be appreciated that if the        workpiece is not sufficiently rigid, more than three net contact        blocks may be employed to reduce or eliminate deformation of the        workpiece.

Each locator 32 is disposed at a corresponding one of the points onwhich the workpiece rests. The locators 32 can comprise rest buttons orpads, concentric locators and radial locators. Clamps 38 can be employedto secure the workpiece to the positioning fixture 20 to thereby inhibittranslation of the workpiece relative to the positioning fixture 20along the X, Y and Z-axes, as well as rotationally about the X-axis in asecond rotational direction.

If desired, the locators 32 can be movably coupled to the positioningfixture 20 so as to be capable of being used for the production ofdifferent finished articles. For example, the locators 32 could bepositionable in a first orientation to facilitate the joining ofworkpieces for a first finished article (e.g., the body-in-white of asedan) and a second orientation to facilitate the joining of workpiecesfor a second finished article (e.g., the body-in-white of asport-utility vehicle). The positioning of the locators 32 can beaccomplished manually with assistance of results from the optimizationalgorithm, or preferably in an automated manner via an appropriatemechanism, such as one or more linear motors (not shown). Moreover, insituations where the locators 32 are moved in an automated manner, thepositioning of the locators 32 can be controlled on an as-needed basis,so that workpieces for a variety of different finished articles could beprocessed together through the manufacturing system 10 (FIG. 1) withoutthe need for significant down-time to re-tool the manufacturing system10 (FIG. 1).

With reference to FIGS. 2 and 3, one or more of the locators 32, whichcan be a concentric locator or a radial locator, can be a primarylocator 32 p. In the particular example provided, the primary locator 32p is a concentric locator and comprises a pin 40, as well as first,second and third linear motors 42, 44 and 46, that are driven bycontroller 24 through associated motor driver and power supply circuitry25 and configured to translate the pin 40 along the X, Y and Z-axes,respectively, relative to the frame 30. It will be understood that themotor of the first, second and third linear motors 42, 44 and 46 can becoordinated to avoid binding the workpiece or breaking the locators. Asis schematically depicted in FIG. 3, the pin 40 can optionally befixedly coupled the one or more other locators 32 in a given set oflocators (i.e., a “first set of locators” in the particular exampleprovided) so that operation of the first, second and third linear motors42, 44 and 46 can optionally move all or a portion of the locators 32 inthe particular set of locators (i.e., as a group) along the X, Y andZ-axes. The first, second, and third linear motors 42, 44 and 46 aredepicted as employing jack- or ball-screws in the particular exampleprovided, but it will be appreciated that any type of linear motor couldbe employed, including those driven by servo and/or stepper motors.

In some situations, the positioning fixture 20 can be configured to havea quantity of primary locators 32 p that is one less than the quantityof workpieces that are to be joined in the workstation 12, and each ofthe primary locators 32 p can be associated with a different set oflocators. It will be appreciated, however, that the quantity of primarylocators 32 p employed in a particular positioning fixture 20 can bevaried as desired. For example, a single primary locator 32 p could beemployed if movement of one type (e.g., translation along the X, Yand/or Z axes) was desired, while two primary locators 32 p could be ifmore than one type of movement (e.g., translation and rotation) wasdesired.

It will be appreciated that the purpose of the primary locators 32 p isto permit the set of locators 32 that control the positioning of oneworkpiece to be moved relative to the set of locators 32 that controlthe positioning of a second workpiece. Optionally, as shown in FIG. 4,the primary locator 32 p can include a pair of retractable clamping jaws50 that are movable between a retracted position, which permits anassociated workpiece W1 to be loaded onto the primary locator 32 p, andan extended position that can be employed to urge the workpiece W1against a shoulder 52 or rest button to thereby fix the workpiece W1against the shoulder 52 or rest button to inhibit translation along androtation of the workpiece W1 about (the pin 40) of the primary locator32 p.

Measuring Device(s)

Returning to FIG. 1, the measuring device 22 can be any type of devicethat is configured to collect data in real time relating to the physicalpositioning of various features on the workpieces when the workpiecesare mounted on the positioning fixture 20 prior to the commencement of ajoining operation. For example, the measuring device 22 can comprise afirst device, such as a laser radar device or an optical measurementdevice, which is configured to collect 3-dimensional data relating tothe workpieces, and an analyzing tool that can be employed to evaluatethe 3-dimensional data and identify the size, shape and relativeposition of selected features on the workpieces.

In one contemplated embodiment, the measuring device 22 includes a laser(not shown), which can be in the form of a laser tracker (not shown),and initially collects data associated with the Z-axis of the first andsecond workpieces or a proxy thereof. This data is employed to “flatten”the first and second workpieces relative to a reference plane that isparallel to the X- and Y-axes. The “flattening” could be accomplished bygenerating an inverse matrix of values (i.e., Z values) along the Z-axisand moving the first and/or second workpieces as necessary to positionthem in a desired manner relative to the reference plane. Thereafter,the measuring device 22 could be employed to collect full 3-dimensionaldata that is employed to determine if the several features on the firstand second workpieces are in their optimized location.

Performance of the method of the present disclosure in this manner maybe beneficial in some situations (e.g., improve the ability of themeasuring device 22 to collect full 3-dimensional data by presenting thefirst and second workpieces in a relatively consistent manner relativeto the reference plane, reduce the time to calculate the optimizedlocation(s), and/or reduce the complexity of the calculations used todetermine the optimized location(s)).

It will further be appreciated that inspection stations could beemployed to acquire 3-dimensional data on each of the first and secondworkpieces prior to their introduction into workstation. In itsinspection station, the workpiece can be loaded into a fixture andclamped into place. A laser could be employed to collect Z-axis dataused to “flatten” the workpiece as described above. Thereafter, ameasuring device could be employed to collect 3-dimensional data of theworkpiece.

As an alternative to measuring the workpiece at the workstation in realtime, the workpiece can be measured in advance and its measurement dataprovided to the optimization algorithm when the workpiece is delivered.For example, the workpiece can be associated with a unique serial numberthat permits it to be associated with a data set that includes its3-dimensional data. Alternatively, a read-write device, such as a radiofrequency tag, can be coupled to the workpiece or a fixture/rack intowhich the workpiece is mounted and the 3-dimensional data for theworkpiece can be written or saved to the read-write device. Data on theread-write device can be accessed from the read-write device when theworkpiece or fixture/rack is sequenced into the assembly process.

The ability to gather 3-dimensional data on workpieces prior toplacement of the workpieces in the assembly station a) permitsnon-conforming workpieces to be identified (and potentially removed orrepaired) prior to entering the assembly workstation, and/or b) has thepotential to reduce the complexity of the 3-dimensional data acquisitionin the assembly workstation (because the 3-dimensional data of theworkpiece is already known, only three points on the workpiece areneeded to understand the orientation of the workpiece in the assemblyworkstation).

It will be appreciated that the “inspection” function could be performedat a separate station that feeds the workpieces to an assembly station,or could be performed at a location that is remote from the assemblyline. For example, if the assembly line/assembly workstation is disposedin an assembly plant of an automobile manufacturer, the inspectionstation for the workpiece could be located at the facility of a supplierthat manufactures the workpiece for the automobile manufacturer.

Analyzing Tool

In the particular example provided, the analyzing tool employs data fromthe first device in conjunction with data transformation techniques andpattern recognition techniques to identify one or more of the selectedfeatures. Each of the features can comprise a surface or edge of aworkpiece, a datum on a workpiece, a hole or slot in a workpiece, etc.and is selected for its ability to influence variation in the finishedarticle. In the particular example provided, the analyzing tool isemployed to a) determine the magnitude of variances between actualfeature dimensions (size, location, etc.) and associated nominal featuredimensions (as determined from blueprints or CAD data), b) determine ifany of the actual feature dimensions is out of tolerance, and c)statistically analyze the magnitude of the variances to determine if theactual feature dimensions are in statistical control or out ofstatistical control. The statistical analysis can be employed toidentify instances where one or more features are being manufactured ina non-ideal manner so that corrective action can be implemented toensure that workpieces subsequently fed into the manufacturing system 10are less apt to add significant variation into the finished article. Itwill be appreciated that the non-ideal manner of manufacture could bethe manufacture of the feature in an out-of-tolerance manner, or couldbe the positioning or forming of the feature at a position or to a sizethat deviates from its nominal blueprint location or size.

Optionally, the measuring device 22 can be employed to identify featuresand/or components of the workpiece(s) and/or assembly that can beout-of-tolerance or otherwise non-conforming (e.g., incomplete orimproperly assembled/fabricated) and generate an appropriate response,such as an alarm, flag or shut-down command, which can be used toprevent the out-of-tolerance/non-conforming workpiece or assembly frombeing used.

The workstation controller 24, or an auxiliary computer (not shown inFIG. 1) communicating with the controller 24, can receive data andinformation from the measuring device 22 and can employ the optimizationalgorithm to re-orient one or more of the workpieces relative to theother workpieces as desired. In this regard, the results of theoptimization algorithm can be employed to operate the first, second andthird linear motors 42, 44 and 46 (FIG. 3) to move one or more of thelocators 32 in a desired manner.

Optimization Algorithm

Generally speaking, the optimization algorithm can determine two or morevectors that can be employed to control the movement of a workpiece toan optimized location. For example, two vectors can be employed where afirst vector corresponds to rotation of the workpiece about an axis, anda second vector corresponds to translation of the workpiece in a plane.It will be appreciated that these two vectors could be employed inseparate movements (i.e., sequentially) or may be combined in somesituations so that rotation and translation corrections could beimplemented simultaneously. The optimization algorithm can alsocoordinate the movement of the primary locators 32 p for a givenworkpiece to prevent binding of a workpiece on a fixture or the breakingof one or more of the locators 32, and can perform a mapping functionthat identifies the (new) position of one or more of the features afterthe primary locator(s) 32 p have moved the workpiece(s).

More specifically, the optimization algorithm is implemented by aprocessor, such as a processor within the controller 24, or by aprocessor within an auxiliary computer that communicates with thecontroller 24. The processor 80, shown diagrammatically in FIG. 11 hasassociated computer-readable memory 82 that is configured according topredefined data structures designed to hold the supplied computer-aideddesign (CAD) data, the workpiece measurement data, and other calculateddata used by the optimization algorithm 84, shown.

While different data structure implementations are possible, the datacan be beneficially allocated to a plurality of data tables, three beingillustrated in FIG. 11: assembly table 86, workpiece table 88 andfeature table 90. In most manufactured articles there is a one-to-manyrelationship (1 . . . n) between a record in the assembly table, and theconstituent workpiece component records stored in the workpiece table.In other words, an assembled article is typically made up of one or moreworkpiece components. Similarly there is typically a one-to-manyrelationship (1 . . . n) between each workpiece and the individualfeatures found on that workpiece. In other words, each workpiece willtypically have one or more features, such as edges, corners, locatingholes, surface features and the like.

FIG. 12 shows the exemplary data structure in greater detail. It will beappreciated that FIG. 12 is intended merely to aid in understanding theoptimization algorithm and is not intended to be an exhaustive anddetailed representation of all data that might be stored in an actualimplementation. The assembly table 86 stores, for each assembly orarticle to be manufactured, a collection of records representing theconnected relationships among the plurality of workpiece components thatmake up the finished assembly or article. These connected relationshipdata can be obtained, for example, by ingesting data from thecomputer-aided design (CAD) files 92 as developed by product engineersof the assembly or article. As another alternative, the connectedrelationship data could be obtained from a master component that isconsidered to dimensionally accurate. To allow the assembly table toaccommodate multiple different articles (e.g., different models ordifferent products), the records may each be tagged with an assembly IDthat is unique for that article. Thus the assembly table can storeplural sets of connected relationship data for a plurality of differentarticles or products.

While the actual representation of each connected relationship may varybased on the implementation, to illustrate the concept, FIG. 12 showsthe connected relationship may be represented as aconnected-relationship string that identifies a feature location on afirst workpiece that mates with a feature location on a second adjoiningworkpiece. In FIG. 12, feature F001 of workpiece W001 mates with featureF001 of workpiece W002, as at 96. Thus, the connected-relationshipstring may contain the following representation of this adjoiningrelationship as the following ordered pair: (W001.F001, W002.F001).Again, it will be understood that this particular ordered pairrepresentation is intended merely to illustrate the concept of how twofeatures can be designated as being mated or proximate features in afinished article. Other representations are also possible.

The workpiece table 88 stores a list of the workpieces required toassemble a finished assembly or article. Thus workpiece table 88 storesrecords that link the assembly ID to a workpiece ID. The workpiece table88 thus serves as a linking table to correlate information stored in theassembly table 86 with information stored in the feature table 90. Thefeature table 90 assigns a unique feature ID to each feature andassociates that feature to its corresponding workpiece, via theworkpiece ID.

The feature table 90 is initially populated with actual measurement data98, obtained using cameras, laser measurement systems and the like.Specifically, from actual measurements taken of an individual workpiece,the relevant features (e.g., points, edges, holes, surface features) areextracted using machine vision or image analysis and the locations ofthese relevant features are stored as measured coordinates in thefeature table 90. Note that these measured coordinates can be expressedwith respect to a reference frame associated with the workpiece beingmeasured. Alternatively, if desired, the measured coordinates can beexpressed with respect to a reference frame associated with theworkstation's positioning fixture into which the workpiece is or will bemounted.

In addition to the measured coordinate data, the feature table 90 isalso populated with a weighting value, for each feature (i.e. eachrecord) in the feature table. These weighting values are used by theoptimization algorithm to control which feature-to-feature relationshipsneed to be tightly constrained, and which can be relaxed. By allowingweighted control over which relationships dominate the assembly, theprocessor is able to calculate an optimal assembly solution thatrespects the design engineers' overall vision for the assembled article.Car doors fit properly; engine mounts balance the engine for minimalvibration; less critically positioned components take up the slack.

The feature table 90 also stores calculated coordinates for each of thefeatures. Initially these calculated coordinate data are unpopulated.The optimization algorithm uses these calculated coordinates storagelocations to store the intermediate and ultimately the final calculatedvalues where each of the features need to be in the final optimizedassembly solution.

The feature table 90 stores all features that are pertinent to theassembly. These features include all feature points or structures thatneed to mate with the feature points or structures of adjacent workpieceparts. These features also include all locator pins, locator holes andother locator surfaces that are used to line up the workpiece with themating locator found on the positioning fixture. Thus, in addition toworkpiece feature data, the feature table also stores feature locationdata of the positioning fixture or fixtures used during the assemblyprocess. This includes the location of all locator pins, or otherlocator structures found on the positioning fixture.

If not already expressed relative to a common reference system, theoptimization algorithm performs any necessary coordinate translation sothat all coordinate stored as calculated coordinates are expressedrelative to a common reference system, such as the reference system ofthe positioning fixture. In this regard, typically one locator withinthe positioning fixture is designated as the primary locator. Thisprimary locator is typically held stationary (i.e., not adjusted by theprocessor controlled linear motors) and may serve as the origin point(0,0,0) of the common reference system.

Referring to FIG. 11, the processor 80, accesses the data structureswithin memory 82 and performs the optimizing algorithm 84 that appliesthe following steps:

-   -   1. Designate the origin of the common reference frame about the        fixture and store its location in the feature table as the        origin (0,0,0). It will be understood that the reference frame        may be in a different position. For example, the global        coordinate system for a car can be placed outside the boundary        of the fixture and in such a case, the algorithm can take that        into account and can map a transformation matrix of the        coordinates into the common fixture frame (i.e., pin locators).    -   2. Represent the coordinates of the positioning fixture locators        as variables (to be determined) in the feature table.    -   3. Express all measured coordinates in the common reference        frame; preferably this common reference frame is that of the        workstation positioning fixture. This can be accomplished by        either acquiring the measured coordinates using a camera system        or measuring device that is calibrated to the common reference        system. Alternatively, this can be accomplished by performing        matrix transformation of the measured coordinates in workpiece        reference frame into the common reference frame.    -   4. Using the connected-relationship data stored in the assembly        table (e.g., ingested from the CAD data), construct a matrix of        ordered pairs to represent each pair of mating features of the        assembly and store as initial calculated coordinates in the        feature table. In the constructed matrix, each calculated        coordinate is represented by a test vector of yet to be        determined length and direction, the vector having its head        coincident with one of the mating features and its tail on the        other of the mating features. See FIG. 13.    -   5. Through an optimization process using priority-based        numerical optimization that takes the stored weighting values        into account, computationally iterate through one or more        iterations to computationally seek the best fit, giving        precedence to points of higher weight, such that the lengths of        all test vectors are minimized. It's worthy to note this        algorithm is not an averaging algorithm. It has the ability to        prioritize features based on importance to the build process and        the end-user.    -   6. When the best fit is found, solve for the positions of all        movable locators or positioning devices (e.g., robotic arms).    -   7. Use the calculated positions of the moveable locators and        positioning structures to drive the linear motors to adjust the        locators and positioning devices in physical space.

FIGS. 13 and 14, show how the optimization algorithm exploits the testvectors to seek the optimal solution. In FIG. 13, three exemplaryworkpieces, W001, W002 and W003 are laid out in virtual space, withtheir respective features (in this case represented by small circles)are aligned approximately adjacent to each other. Between each pair ofmating features, a test vector 100 is defined stored in virtual spacewithin processor memory 82. Because each workpiece is represented in thecommon coordinate frame, each of these test vectors is referenced to theorigin point of the coordinate system. For this example, we shall assumethat workpiece W002 is oriented on the positioning fixture by beingphysically attached to the primary locator 32 p, which serves as theorigin (0,0,0). Each other feature remains of unknown position until theoptimization algorithm is run. This includes the position of locator 32v, which corresponds to one of the motor-driven variable locators.

When the optimization algorithm is run, a solution is calculated thatminimizes the length of all of the test vectors (by utilizing the3-dimensional measured data and ideal data), taking individual weightingof each pair of mating features into account. The effect is to determinethe final location of all features in the optimal arrangement. This hasbeen diagrammatically illustrated in FIG. 14. FIG. 14 at 102 shows thecondition of three exemplary test vectors a, b and c before anyoptimization is performed. The test vectors each have a direction andlength sufficient to connect its respective pair of mating points.Because no optimization has been performed at this stage, these testvectors may be of any length and of any direction.

As the priority-based numerical optimization algorithm 106 isrun—possibly iteratively or recursively a number of times—the optimallengths and directions of test vectors a, b and c are ultimately arrivedat as shown at 104. Note in this example, test vector b has receivedhighest weighting priority, resulting in its associated pair of matingpoints now being coincident. The other test vectors a and c have beenshortened in length, although not as much as vector b. Note that forthis particular example test vectors a and c have received adjustment inpointing direction as well.

Essentially, the optimized change in length and direction of the testvectors corresponds to a shift in the three-dimensional position androtation of the workpieces to which the corresponding features areassociated. Once the optimized solution (illustrated in 104) isachieved, the optimized positions of all features are fixed in referencecoordinate space. That is, the position of each feature can bedetermined by first establishing the locations relative to the origin(0,0,0) of the workpiece associated with the primary locator 32 p (inthis case workpiece W002). Then the locations of the mating points onother adjacent workpieces are determined by vector addition, using theoptimized test vectors.

As noted above, once the best fit for the workpieces have beendetermined in virtual space, the process can now solve for the requiredcoordinates of the moveable locator pins and either automatically movethose pins to proper locations, or provide detailed instructions toassist a worker in doing so.

Having thus presented an explanation of the optimization algorithm, useof the overall system can now be discussed.

Joining Workpieces Using Variable Position Locators

With reference to FIG. 5, an exemplary method for performing a joiningoperation at a given workstation 12 (FIG. 1) is schematically depicted.The method can begin at step 60, where a first workpiece and a secondworkpiece are mounted on first and second sets of locators 32 a and 32 b(FIG. 1), respectively. At this initial stage, each of the primarylocators 32 p (FIG. 1) associated with the first set of locators 32 a(FIG. 1) is positioned relative to the second set of locators 32 b(FIG. 1) by an associated set of the first, second and third linearmotors 42, 44 and 46 (FIG. 3) at an initial position. Clamps (notspecifically shown) can be operated to secure the first and secondworkpieces to the frame 30 (FIG. 1).

The method can proceed to step 62, where the measuring device 22(FIG. 1) can collect and analyze 3-dimensional data regarding selectedfeatures on the first and second workpieces. In the particular exampleshown in FIG. 6, which depicts a door D for an automotive vehicle, thefeatures comprise first and second pairs of hinge mount holes HM1 andHM2, respectively, a pair of latch mount holes LM, and a datum line DL.

Returning to FIGS. 1 and 5, the method can proceed to decision step 64where control (e.g., the workstation controller 24) can determine if theseveral features are in their optimized location. If not, the processproceeds to step 66 where the optimization algorithm is performed.Specifically, if the features are not in their optimized locations, themethod can proceed to step 66 where the primary locator 32 p (with theassociated set of locators 32) can be moved by the first, second andthird linear motors 42, 44 and 46 (FIG. 3) as required to position thefirst workpiece at a location that positions the features on the firstand second workpieces at their optimized locations. The method can loopback to step 62, which permits the measuring device 22 to confirm thepositioning of the features in their optimized locations (withinpredefined limits) and to permit the workstation controller 24 toconfirm that the optimized locations have not changed.

If at decision step 64 the features are in their optimized locations,the method can proceed to step 68 where the workpieces can be securedtogether. In the example provided, the workpieces are joined via a MIGwelding process, but as noted above, other joining processes could beemployed in the alternative. The method can proceed to step 70,discussed in the Assessing Manufacturing Tolerances and RefiningManufacturing Process section below.

Further Discussion of Weighting Function

For purposes of this methodology, an optimized location of the featuresis the positioning of the first and second workpieces in a manner thatminimizes the effect that the features of the first and secondworkpieces have on the magnitude of the variation in the fabrication ofthe article. Note that the optimized location of the features is notnecessarily the location that minimizes the variation between thenominal location of each feature and the actual location of eachfeature, as in a least squares regression analysis. In our experience,the several features will have differing levels of influence on themagnitude of variation in the fabrication of the article andconsequently, thus the processor employs a weighting technique withinthe optimization algorithm, as discussed above.

To illustrate how weighting comes into play, refer to the example ofFIG. 6. In FIG. 6 a pair of hinges (not shown) shall be mounted to thedoor D via first and second pairs of hinge mounting holes HM1 and HM2.Further, a latch (not shown) shall be mounted to the door D via the pairof latch mount holes LM. Some of these holes may be configured with arelatively large degree of compliance (e.g., the holes in a first one ofthe hinges and the latch that receive fasteners that are threaded intothe second hinge mount holes HM2 and latch mount holes LM are relativelylarger in diameter than the fasteners). The other one of the hingesshall be mounted to the door D via the first pair of hinge mount holesHM1 and is configured with a relatively smaller degree of compliance(i.e., the holes in the other one of the hinges is sized relativelycloser to the threaded fasteners that are threaded into the hingemounting holes HM1). According to the desired end result, the datum lineDL is critical to the gap-and-flush fit of the door D to the remainderof the body (not shown) of the automotive vehicle. In this example, thedatum line DL is given a first weight (e.g., a weight of one (1.0)), thefirst hinge mount holes HM1 is given a second, smaller weight (e.g., aweight of one-half (0.5)), while the second pair of hinge mount holesHM2 and the latch mount holes LM are each given still smaller weights(e.g., a weight of one-tenth (0.1)). Accordingly, the several featurescan be prioritized in the optimization algorithm such that the featureor features that most influence the magnitude of variation in thearticle can be oriented as close as possible to their nominal positionsto thereby reduce the magnitude of variation in the article.

In some situations, the weighting could cause one or more of thefeatures to be moved away or further away from their nominalposition(s). In the example of FIG. 7, the latch mount holes LM areoriginally located at their nominal locations, but due to the priorityassociated with the location of the datum line DL, the workpiece W1 canbe moved somewhat to re-orient the datum line (to the position that isdepicted in broken line), which can move the hinge mount holes HM (tothe positions shown in broken line) away from their nominal positions.In severe instances, one or more lower weighted features could actuallybe moved into a position that is actually considered out-of-tolerancebut which nevertheless permits the first and second workpieces to bejoined and functionally integrated into the article. For example, ifsecond pair of hinge mount holes HM2 were to be positioned within +/−0.5mm from their nominal position, but were positioned +0.6 mm from theirnominal position due to the prioritization of the datum line DL, theoptimization algorithm would permit the configuration if there wassufficient compliance between the hinge and the second pair of hingemount holes HM2 to permit the door D to be mounted to the remainder ofthe vehicle body such that the door D opened, closed and latchedproperly and all criteria associated with the gap-and-flush fitting ofthe door D to the remainder of the vehicle body could be satisfied. Onekey to obtain the maximum benefit of the optimization algorithm is toappreciate that some tolerances may be somewhat arbitrary and that anout-of-tolerance situation for one feature does not necessarily renderthe article defective or inoperative. It will be appreciated, however,that limits could optionally be placed on the optimization algorithmthat would not permit the location of one or more features to bepositioned at an out-of-tolerance position.

Virtual Build Techniques

The optimization algorithm can be used to optimize the locations offeatures on adjoining workpieces by taking into account not only thepositions of features on the workpieces to be joined at a particularworkstation, but also the positions of features on workpieces that willbe added at a later manufacturing stage, or at a different workstation.Thus, for example, although only three workpieces and eight featurepoints, plus locator pins are considered in the example of FIG. 13, theoptimization process performed to shorten the test vectors 100 can takeinto account constraints imposed by other workpieces that are notcurrently being joined at this workstation. Thus for example, workpieceW001 may have another set of features used to join that workpiece toanother workpiece (not shown). The optimization algorithm can take theseother features into account in arriving at the solution for where tolocate the variable pin 32 v.

If desired, the entire product, made up of many components,subassemblies and workpieces can be modeled and used as data for theoptimization algorithm. Doing this will help ensure that all of thecomponents will ultimately fit together optimally, even if the joiningis performed at different workstations.

In essence, one can consider every manufacturing process as comprising avirtual build. In the simple case, the entire virtual build is optimizedusing the optimization algorithm and then used to perform the physicalbuild at a single workstation. Two parts might be joined together inthis fashion. In the complex case, the virtual build is optimized,possibly for many hundreds of features, and the results are used toperform the physical build at multiple workstations, operating in serialor parallel, or both.

When using virtual build techniques involving many components,subassemblies and workpieces, and involving multiple workstations orbuild operations, the optimization process may be adapted to account forvariability that may creep in as individual subassemblies are joined. Todo this, the measuring devices 22, discussed above, can be used to takefresh measurements to determine the actual location of features ofsubassemblies after they have been joined. These are stored in thefeature table and used when performing the optimization algorithm.

Considering Whether Workpieces are Independent or Dependent

With reference to FIGS. 8 and 9, another factor in maximizing thebenefit of the optimization algorithm concerns identifying ordetermining whether the workpieces that are to be joined are“independent”, which is shown in FIG. 8, or “dependent”, which is shownin FIG. 9. In both examples, two sets of locators (not specificallyshown) are employed, with a first set of the locators being employed toposition a first workpiece W1 and a second set of the locators beingemployed to position a second workpiece W2. In these situations only oneprimary locator (not shown) is required, but it will be appreciated thatone or more additional primary locators could be integrated into thefirst set of locators and/or the second set of locators.

With reference to FIG. 8, an independent situation is depicted in whichthe features on one workpiece (e.g., the features Fa, Fb, Fc and Fd onthe first workpiece W1) are able to be positioned relative to thefeatures on a second workpiece (e.g., the features Fg, Fh, Fi, Fj and Fkon the second workpiece W2) without causing movement of the secondworkpiece W2. In the example provided, a portion of the first workpieceW1 overlies a portion of the second workpiece W2 and the two workpiecesW1, W2 are joined together via a lap weld. The first workpiece W1 can bepositioned independently of the second workpiece W2 so that the featuresFa, Fb, Fc and Fd can be collectively oriented in a desired mannerwithout movement of the second workpiece W2 or the features Fg, Fh, Fi,Fj and Fk.

With reference to FIG. 9, a dependent situation is depicted in whichindependent movement of the features on one workpiece (e.g., thefeatures Fa, Fb, Fc, Fd, Fe and Ff on the first workpiece W1) relativeto the features on a second workpiece (e.g., features Fg, Fh, Fi, Fj,Fk, Fl and Fm on the second workpiece W2) is limited by an interactionbetween at least one of the features on the first workpiece W1 and atleast one of the features on the second workpiece W2. In the exampleprovided, a portion of the first workpiece W1 overlies a portion of thesecond workpiece W2 and the first and second workpieces W1, W2 aresecured to one another via a lap weld. Unlike the prior example,however, the features Fe and Ff on the first workpiece W1 and thefeatures Fl and Fm on the second workpiece W2 are holes that are to bealigned to one another to receive threaded fasteners (not shown)therethrough that permit the joined first and second workpieces W1 andW2 to be mounted to a portion of the vehicle body. In this situation,movement of the first workpiece W1 relative to the second workpiece W2is limited by a number of factors, including the amount of clearancebetween the holes and the threaded fasteners and the location of theholes in the vehicle body that receive the threaded fasteners.

Data Feed-Forwarding

Data regarding the optimized locations can be employed for feedforwardand trend analysis. Feedforwarding of the data permits coordinatesassociated with the optimized locations to be fed between stationsand/or to re-set the “home” position of a station.

While the method has been described as comprising a series of discretesteps that are performed sequentially, it will be appreciated that themethod could be performed somewhat differently. For example, portions ofthe method described above in conjunction with steps 62 through 66 couldbe intermixed and/or performed at multiple stations.

Assessing Manufacturing Tolerances and Refining Manufacturing Process

With reference again to FIG. 5, although the workpieces have beensecured at step 68, the process may continue through steps 70-76 inorder to gather data on and assess manufacturing tolerances, as an aidto refining and improving the overall manufacturing process.

Thus in step 70 the measuring device 22 can be employed to determine thelocations of the features. The method can proceed to decision step 72.

In decision step 72, control determines whether the features are intheir optimized locations within predefined tolerances. If the featuresare not in their optimized locations within the predefined tolerances,the method proceeds to step 74 where the assembly (i.e., the joinedworkpieces) are identified as being non-compliant. Such pieces may bescrapped or reworked as necessary.

Returning to decision step 72, if the features are in the optimizedlocations with the predefined tolerances, the method proceeds to step 76where the method ends and all collected data are stored for laterprocess improvement. The assembly is considered to be compliant withtolerances and can be fed into a subsequent workstation as part of thesubsequent steps for fabricating the article. It will be appreciatedthat the positions of the primary locators 32 p can be returned to a“home” or “nominal position” after the joined assembly has been removedfrom the fixture. Optionally, the positions of the primary locators 32 pcan be left at their current positions.

Multi-Step Fabrication Using Plural Workstations

With reference to FIG. 10, a first plot P1 and a second plot P2 areemployed to illustrate variation created in a prior art multi-stepfabrication process and a multi-step fabrication process in accordancewith the teachings of the present disclosure, respectively. A prior artsystem for fabricating an article is configured in a manner that seeksto position each pair of workpieces that are to be joined at predefinedpositions (e.g., their nominal positions) prior to the joiningoperation. Fabrication of the article in this manner produces variation(due to deviations in the tooling that is employed to fixture and orientthe workpieces, as well as deviations in the workpieces themselves) thatis compounded at each subsequent step of the fabrication process asdepicted by the first plot P1 and as such, the magnitude of thevariation that is possible in the finished article can be relativelylarge if there are many steps to the fabrication process and/or if thearticle is relatively complex in its configuration, such as abody-in-white. In contrast, the fabrication system of the presentdisclosure seeks to position each pair of workpieces that are to bejoined in a manner that minimizes the magnitude of variation in article.In this regard, the several workpieces in a workstation can bepositioned in optimized locations to eliminate variation caused bydeviations in the tooling that is used to fixture and orient theworkpieces, as well as to reduce variation caused by deviations in theworkpieces themselves. Accordingly, variation created in one workstationcan be attenuated in a subsequent station as shown in the second plot P2and consequently, the magnitude of variation that is possible in thefinished article is significantly reduced as compared to the prior artmulti-step fabrication process.

It will be appreciated that the location optimizing technique that isemployed in the workstations of the manufacturing system 10 (FIG. 1) caneliminate the effect of tooling wear on the fixturing that is used toposition the workpieces, can eliminate the production of assemblies thatare not fit for use in the finished article through real-time monitoringof the features of the workpieces before the joining operation, canreduce scrap by permitting the use of some workpieces having one or moreout-of-tolerance features, and can improve the overall quality of thefinished article relative to a conventional joining operation that seeksto orient workpieces in their nominal positions prior to joining theworkpieces.

While the manufacturing system 10 (FIG. 1) has been described asemploying a plurality of workstations 12 (FIG. 1), each having apositioning fixture 20 (FIG. 2) that includes a jig or a frame 30 (FIG.2) and at least one primary locator 32 p (FIG. 2), it will beappreciated that a portion of the jig or frame 30 (FIG. 2) could becoupled to or integrated with a robot (i.e., the end effector of arobot) and that the robot could be employed to vary the position of anassociated workpiece in 3-dimensional space as needed. Configuration inthis manner essentially integrates a set of the locators 32 (FIG. 2)with portion of the jig or frame 30 (FIG. 2) so that the motors that areemployed to position the robot are substituted for the linear motors andmove (as a group) the portion of the jig or fixture and an associatedset of locators.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A method for forming a multi-piece assembly in aplurality of joining operations that progressively joins workpiecestogether, each of the workpieces having one or more features, themulti-piece assembly having an assembly specification that definestarget positions of the features of the workpieces relative to a datum,the method comprising: securing first and second workpieces to at leasta first assembly fixture; determining an initial positioning between thefirst and second workpieces that relates a first three-dimensional dataset of the first workpiece to a second three-dimensional data set of thesecond workpiece, the first three-dimensional data set includingthree-dimensional data on each of the features on the first workpiece,the second three-dimensional data set including three-dimensional dataon each of the features on the second workpiece; determining a desiredrelative positioning between the first and second workpieces based atleast partly on the first and second three-dimensional data sets and theassembly specification, wherein the desired relative positioning isspecific to a given pair of the first and second workpieces and isconfigured to locate each of the features of the first and secondworkpieces in such a way that discrepancies in the positioning of all ofthe features on the first and second workpieces relative to theirpositioning in the assembly specification are minimized in a desiredmanner; while the first and second workpieces are secured in the atleast a first assembly fixture, repositioning the first workpiecerelative to the second workpiece such that the first and secondworkpieces are in the desired relative positioning; and joining thefirst and second workpieces together.
 2. The method of claim 1, whereinthe at least the first assembly fixture comprises a primary locator thatengages at least one of the first and second workpieces.
 3. The methodof claim 2, wherein the primary locator is a pin.
 4. The method of claim2, wherein the at least the first assembly fixture comprises a frame andwherein the primary locator is movable relative to the frame.
 5. Themethod of claim 4, wherein the primary locator is movable in each ofthree orthogonal axes.
 6. The method of claim 1, wherein the at leastthe first assembly fixture comprises a robot having an end effector thatengages at least one of the first and second workpieces.
 7. The methodof claim 1, further comprising converting a set of measurement data forthe first workpiece into the first three-dimensional data set.
 8. Themethod of claim 7, further comprising operating a measurement device togenerate the set of measurement data for the first workpiece.
 9. Themethod of claim 1, wherein the joined first and second workpieces form asub-assembly and wherein the method further comprises: securing thesub-assembly and a third workpiece to at least a second assemblyfixture; determining an initial positioning between the sub-assembly andthe third workpiece that relates the first and second three-dimensionaldata sets to a third three-dimensional data set of the third workpiece,the third three-dimensional data set including three-dimensional data oneach of the features on the third workpiece; determining a desiredrelative positioning between the sub-assembly and the third workpiecebased at least partly on the first, second and third three-dimensionaldata sets and the assembly specification, wherein the desired relativepositioning between the sub-assembly and the third workpiece is specificto a given pair of the sub-assembly and the third workpiece and isconfigured to locate each of the features of the third workpiecerelative to the features of the sub-assembly in such a way thatdiscrepancies in the positioning of all of the features on the first,second and third workpieces relative to their positioning in theassembly specification are minimized in a desired manner; while thesub-assembly and the third workpiece are secured to at least the secondassembly fixture, repositioning the third workpiece relative to thesub-assembly such that the sub-assembly and the third workpiece are inthe desired relative positioning between the sub-assembly and the thirdworkpiece; and joining the sub-assembly and the third workpiecetogether.
 10. The method of claim 9, wherein the desired relativepositioning between the sub-assembly and the third workpiece isconfigured to attenuate variation in the multi-piece assembly that isrelated to the joining of the first and second workpieces together whenthe first and second workpieces were positioned at the desired relativepositioning between the first and second workpieces.
 11. The method ofclaim 1, wherein the desired relative position is further determined atleast partially based on a weighting technique that relates variation inthe positioning of each of the features of the first and secondworkpieces relative to the assembly specification to the impact of thatvariation on the multi-piece assembly.
 12. An assembly line thatperforms the method of claim
 1. 13. A multi-piece assembly constructedaccording to the method of claim
 1. 14. The method of claim 1 whereinthe discrepancies in the positioning of all of the features on the firstand second workpieces relative to their positioning in the assemblyspecification are minimized using a processor implemented numericaloptimization algorithm.
 15. The method of claim 1 wherein thediscrepancies in the positioning of all of the features on the first andsecond workpieces relative to their positioning in the assemblyspecification are minimized using a processor implemented priority-basednumerical optimization algorithm.
 16. A method for forming a multi-pieceassembly by joining mating workpieces in a workstation having at leastone fixed locator and at least one adjustable locator, wherein eachworkpiece has at least one physical feature, the position of whichwithin the multi-piece assembly is expressed in a collection of assemblydata, the method comprising: placing the workpieces in the workstationso that one of said workpieces is in registration with the fixedlocator; obtaining measurement data reflecting the positions of therespective features of the workpieces and representing the measurementdata in a common reference frame associated with the fixed locator;using a processor to ingest and utilize the collection of assembly dataand the measurement data to define and store in memory ordered pairs ofmating features; using the processor to define and store in memory, foreach ordered pair of mating features, a test vector that connects a pairof mating features, where each test vector has vector componentsrepresented as variables stored in memory; using the processor tocomputationally discover a best fit for the mating workpieces bycollectively applying to the test vectors a computational optimizationprocess that seeks to minimize the lengths of the test vectors whiletaking into account predefined weighting factors that give certainfeatures locational preference over others; using the processor toreorient the data representing the feature locations of the respectiveworkpieces in the common reference frame using the computationallydiscovered best fit; using the processor to calculate the position inthe common reference frame of a feature demarked for registration withthe adjustable locator; causing the adjustable locator to move to thecalculated position of the feature demarked for registration with theadjustable locator and thereby establishing a best fit orientation ofthe mating workpieces in physical space; positioning said matingworkpieces in said best fit orientation by registration with said fixedand adjustable locators and then mechanically joining the matingworkpieces.
 17. The method of claim 16 wherein the step of causing theadjustable locator to move is performed using at least one motor orrobot controlled by the processor.
 18. The method of claim 16 whereinthe step of obtaining measurement data is performed optically while theworkpieces are disposed in the workstation.
 19. The method of claim 16wherein the step of obtaining measurement data is performed at least inpart prior to placing at least one of the workpieces in the workstation.20. The method of claim 16 wherein the step of obtaining measurementdata is performed at least in part prior to placing at least one of theworkpieces in the workstation and transformed into said common referenceframe by said processor.
 21. The method of claim 16 further comprisingsupplying said collection of assembly data as computer aided designdata.
 22. The method of claim 16 wherein the computational optimizationprocess employs a numerical optimization algorithm.
 23. The method ofclaim 16 wherein the computational optimization process employs apriority-based numerical optimization algorithm.
 24. The method of claim16 further comprising obtaining second measurement data reflecting thepositions of features of workpieces other than those placed in theworkstation and including said second measurement data in the step ofusing the processor to computationally discover a best fit for themating workpieces.
 25. A method for forming a multi-piece assembly byjoining mating workpieces in a workstation having at least one fixedlocator and at least one adjustable locator, wherein each workpiece hasat least one physical feature, the position of which within themulti-piece assembly is expressed in a collection of assembly data, themethod comprising: placing the workpieces in the workstation so that oneof said workpieces is in registration with the fixed locator; obtainingmeasurement data reflecting the positions of the respective features ofthe workpieces and representing the measurement data in a commonreference frame associated with the fixed locator; using a processor toingest and utilize the collection of assembly data and the measurementdata to define and store in memory ordered pairs of mating featurelocations; using the processor to reorient the pairs of mating featurelocations using a computationally discovered best fit; using theprocessor to calculate the position in the common reference frame of afeature demarked for registration with the adjustable locator; causingthe adjustable locator to move to the calculated position of the featuredemarked for registration with the adjustable locator and therebyestablishing a best fit orientation of the mating workpieces in physicalspace; positioning said mating workpieces in said best fit orientationby registration with said fixed and adjustable locators and thenmechanically joining the mating workpieces.
 26. The method of claim 25wherein the step of causing the adjustable locator to move is performedusing at least one motor or robot controlled by the processor.
 27. Themethod of claim 25 wherein the step of obtaining measurement data isperformed optically while the workpieces are disposed in theworkstation.
 28. The method of claim 25 wherein the step of obtainingmeasurement data is performed at least in part prior to placing at leastone of the workpieces in the workstation.
 29. The method of claim 25wherein the step of obtaining measurement data is performed at least inpart prior to placing at least one of the workpieces in the workstationand transformed into said common reference frame by said processor. 30.The method of claim 25 further comprising supplying said collection ofassembly data as computer aided design data.
 31. The method of claim 25wherein the computational optimization process employs a numericaloptimization algorithm.
 32. The method of claim 25 wherein thecomputational optimization process employs a priority-based numericaloptimization algorithm.
 33. The method of claim 25 further comprisingobtaining second measurement data reflecting the positions of featuresof workpieces other than those placed in the workstation and includingsaid second measurement data in the step of using the processor toreorient the pairs of mating feature locations using a computationallydiscovered best fit.
 34. A method for forming a multi-piece assembly byjoining mating workpieces in a workstation having at least one fixedlocator and at least one adjustable locator, wherein each workpiece hasat least one physical feature, the position of which within themulti-piece assembly is expressed in a collection of assembly data, themethod comprising: obtaining measurement data reflecting the positionsof the respective features of the workpieces and representing themeasurement data in a common reference frame associated with the fixedlocator; using a processor to ingest and utilize the collection ofassembly data and the measurement data to computationally discover abest fit for the mating workpieces in virtual space; causing theadjustable locator to move to a calculated position based on thecomputationally discovered best fit in virtual space and therebyestablishing a best fit orientation of the mating workpieces in physicalspace; positioning said mating workpieces in said best fit orientationby registration with said fixed and adjustable locators and thenmechanically joining the mating workpieces.